How can I use imperial units when entering measurements?
This is now supported, as of this commit (it's in 2.70).

To use this:


Start the modal transform operator (grab, rotate, etc.)
Press = to enter advanced mode
Enter the value you want according to the units you have selected (e.g. for imperial: 4' 11')


When in advanced mode you can do everything you can do in a normal number button:



If you don't press = to enter advanced mode, it should behave mostly as it did before 2.70.

From the wiki:


  We have two "modes", simple and advanced ones, use '=' or 'pad*' to
  enable advanced mode, and 'ctrl-=' or 'ctrl-pad*' to switch back to
  simple mode.
  
  Simple mode works nearly like it did before, it only accepts simple
  numbers, and you can still use '-' to negate and '/' to inverse the
  value (as well as non-number input shortcuts like XYZ, RSG, etc.).
  Compared to Blender 2.69 behavior, you can now copy and paste
  expressions, and navigate in the number you typed, so you can for
  example fix a typo without having to type everything again.
  
  In advanced mode, we gain more power and flexibility, but lose a few
  "shortcuts" like '-' to negate, or '/' to inverse. It features:
  
  
  Units (cm, ", deg, etc.).
  Basic operations from python/BKE_unit (+, *, **, etc.), and math constants and functions (pi, sin, etc.).
  You can navigate in edited value (left/right key, ctrl to move by block) and insert/delete chars, e.g. to fix a typo without having to
  rewrite everything.
  You can copy/paste expression with usual ctrl-C/V.
  You can go to next/previous value with (ctrl-)TAB key.
  As before, hitting backspace after having deleted all leading chars will first reset the edited value to init state, and on second
  press, the whole number editing will be canceled, going back to usual
  transform with mouse.
  You can still use the - and / shortcuts, as well as various transform options (like XYZ, E and F for vertex/edge slide, etc.)
  while in modal numinput mode, you just have to use ctrl to activate
  them!